Utilization of DRI in the steelmaking industry is expanding and the number of so-called mini-mills, which are steel plants of relatively small and medium production capacity, is increasing. These mini-mills comprise one or more direct reduction plants, wherein DRI is produced from iron ores, and electric arc furnaces (EAF) for melting said DRI and producing liquid iron and steel.
The economic and operational advantages of modern steelmaking mini-mills have been recognized in the prior art and some methods and apparatus have been proposed for rendering the mini-mills facilities more efficient with lower investment and operational costs.
DRI is a solid granular material which is produced by reaction of iron ores (mainly iron oxides) in solid phase with a reducing gas at a high temperature of about 900° C. to about 1100° C. in a reduction reactor with or without a DRI cooling zone. DRI is then melted, preferably in an electric arc furnace, to produce molten iron and transformed into liquid steel. Direct reduction plants typically comprise a continuous moving bed reactor discharging hot or cold DRI.
It is known to produce cold DRI in shaft-type moving bed reactors having an upper reduction zone where iron ore particles, as lumps, pellets or mixtures thereof, are reacted with a reducing gas comprising hydrogen and carbon monoxide fed to said reactors at temperatures in the order from about 800° C. to about 1050° C., whereby the hot DRI is then cooled down in a lower cooling zone to temperatures below 100° C. before it is discharged from the reactor in contact with air. The discharge temperature must be below about 100° C. to prevent the DRI from spontaneously reoxidizing. Such reactors producing cold DRI are described for example, in U.S. Pat. Nos. 3,765,872, 4,046,557 and 4,150,972.
It is also known to produce hot DRI in shaft-type moving bed reactors designed to withstand the high-temperatures of DRI, from about 500° C. to about 1000° C., and to promote a gravity-driven mass flow of solids in the reactor, meaning that solid particles move uniformly through the reduction zone in contact with the reducing gas, thus assuring that all particles are chemically reduced to metallic iron to the same degree. This is important for assuring a uniform good quality of the DRI. Examples of hot DRI reactors are described in U.S. Pat. Nos. 3,799,367; 4,188,022; 4,725,309 and 6,214,086.
Applicants have found the following prior art related to the present invention.
U.S. Pat. No. 6,241,804 to Roselfellner describes a process for producing iron briquettes and/or cold iron sponge. This patent shows a shaft-type moving-bed direct reduction reactor 1 where the hot DRI descending from the reduction zone 2 is cooled down in zone 10 by means of a cooling gas system 13. Cold DRI is discharged at a temperature of 30° C. to 100° C. through the product discharge zone 11. Hot DRI, at a temperature of 600° C. to 1000° C. may be produced by extracting it from zone 16 located below the reduction zone utilizing screw conveyors 17. Hot iron briquetting machines 18 are located close to the screw conveyors 17 for producing iron briquettes and be able to store it. Although the general object of producing both types of products, i.e. cold sponge iron and also hot iron briquettes with the same reduction reactor is addressed in this patent, the proposed reactor has a more complicated design and is intended to produce cold DRI most of the time while the hot iron briquettes production is proposed by exception when the normal cold DRI production must be interrupted. This patent is mute regarding the way the screw conveyors are arranged below the reduction zone, which feature is of major importance considering that normally the solid DRI particles flow downwardly through the reactor to be discharged cold, therefore it is concluded that in practice, the screw conveyors will interfere most of the time with the flow of DRI.
U.S. Pat. No. 6,214,086 to Montague et al. describes a method and apparatus for simultaneously supplying varying proportions of hot and cold DRI from a source of hot DRI for melting, storage, briquetting or transport. The system of this patent uses a gravity transport of the DRI and the production of cold DRI is achieved by cooling the hot DRI in a separate cooling vessel outside the reduction reactor which is intended to produce only hot DRI.
U.S. Pat. Nos. 5,296,015 and 5,445,363 to Becerra-Novoa et al. disclose a method for the pneumatic transport of hot or cold DRI as a means for efficiently handling and transporting hot DRI from a reduction reactor to a variety of points of use of such DRI, for example to a DRI melting furnace, a briquetting machine or a storage bin.
Although the above patents suggest the possibility of cooling down the hot DRI within the reduction reactor by means of circulating a cooling gas through the lower portion of the reduction reactor, these patents however do not provide any of the advantages provided by the present invention when the continuous production of hot DRI in a moving bed shaft-type reactor is integrated with a meltshop utilizing one or more batch-type DRI melting devices, for example electric arc furnaces (EAF).
The current practice in design and operation of DRI-based steelmaking plants having normally hot discharge is to provide a separate DRI cooling vessel for cooling the DRI and then allowing for its discharge to open atmosphere for its safe storage or bulk transport. This separate cooling vessel, however, requires an additional bin for cooling (when a discharge cone already exists in the reactor) with all the associated additional piping and hot product handling equipment needed for access to the remote cooling bin, increases the investment and operational costs of a steelmaking plant.
Documents cited in this text (including the foregoing patents), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.
The present invention does not require a separate DRI cooling vessel and is intended to provide a direct reduction reactor system designed for a normal production of hot DRI and exceptionally for cooling down said DRI for storage and later utilization, wherein the change of product from hot DRI to cold DRI and vice-versa is smoothly carried out with minimum disturbances in the operation and production of the integrated steelmaking plant.